The development of surgical techniques have made great progress over the years. For instance, for patients requiring brain surgery, non-invasive surgery is now available which is afflicted with very little trauma to the patient.
One system for non-invasive surgery is sold under the name of Leksell Gamma Knife®, which provides such surgery by means of gamma radiation. The radiation is emitted from a large number of fixed radioactive sources and are focused by means of collimators, i.e. passages or channels for obtaining a beam of limited cross section, towards a defined target or treatment volume. Each of the sources provides a dose of gamma radiation which is insufficient to damage intervening tissue. However, tissue destruction occurs where the radiation beams from all radiation sources intersect or converge, causing the radiation to reach tissue-destructive levels. The point of convergence is hereinafter referred to as the “focus point”. Such a gamma radiation device is referred to and described in U.S. Pat. No. 4,780,898.
In the known system, the head of a patient, immobilized in a stereotactic instrument which defines the location of the treatment volume in the head, is secured in a positioning system which moves the stereo-tactic instrument and the head so as to position the treatment volume in coincidence with the focus point. In other words, only the head of the patient is moved when positioning the treatment volume at the focus point. Thus, the size and the weight of the part to be moved, i.e. the head of the patient along with the stereotactic instrument, is well-defined and within a limited range. Motors for providing said movement can be arranged close to or at the stereotactic instrument, and the tolerances of the positioning system may be kept very small. A drawback of the system is that the patient may experience some discomfort when the head is moved while the body is kept still. In particular when the head is moved during the actual surgery, such as is described in U.S. Pat. No. 5,528,651.
Thus, there is a need for a radiation therapy system for brain surgery, in which the entire patient is moved during positioning of the treatment volume at the focus point. However, such a system would need a positioning system capable of accommodating patients of all sizes, ranging from infants to patients exceeding two meters in length and two hundred kilograms in weight. This would require extremely small tolerances for the constructional parts comprised in the patient positioning system in order to achieve sufficient accuracy in the positioning of the treatment volume at the focus point.
Furthermore, due to the fact that the entire patient is moved and not just the head, the motors and the linear guide system provided for movement of the patient would have to be arranged at a greater distance from the focus point, than with the conventional radiation therapy systems. Typically, the larger distances between the motors, the guideways and the focus point enhances the requirement for smaller tolerances even further. Since improved constructional tolerances normally result in increased production costs, especially when producing a system comprising co-operating moving parts, such a positioning system would be very expensive to produce.